This invention relates generally to digital communication channels for local area networks and, more particularly, to techniques for using unshielded twisted pairs of conductors as a communication medium in a high-speed local area network. High-performance local area networks (LANs) typically use optical fiber or coaxial cable as the communication medium. For example, one token ring network using optical fiber is known as the fiber distributed data interface (FDDI), and is intended for operation at a bandwidth of 125 megabits per second (Mb/s) over distances of about two kilometers. The physical layer medium dependent (PMD) sublayer protocol for FDDI is defined in an American National Standards Institute (ANSI) document designated X3T9.5/84-88. The other sublayer of the physical layer is known as the physical layer protocol (PHY) and is defined in ANSI X3.148-1988.
A significant drawback to LANs using optical fiber as the communication medium is the relatively high cost of the optical fiber. Relatively slow LANs have been proposed using much cheaper unshielded twisted pair wiring. U.S. Pat. No. 5,119,402 entitled "Method and Apparatus for Transmission of Local Area Network Signals Over Unshielded Twisted Pairs," issued in the names of Simon A Ginzburg et al., describes and claims a system in which FDDI signals are transmitted over two or more twisted pairs of conductors, using a modified three-level duobinary code and an arrangement of dumultiplexers and multiplexers. The modified duobinary code does not provide a practical solution to the problems involved with transmitting FDDI signals over a single twisted pair.
FDDI data transmission requirements have a frequency spectrum that quite literally transforms an unshielded twisted pair if conductors into a broadcasting antenna. As a result, satisfying the radiation limitations imposed by the Federal Communications Commission (FCC) is a matter of critical concern in designing a twisted pair communication medium suitable for FDDI signals. Electromagnetic radiation from a twisted pair medium can be reduced by lowering the driving voltage at the transmitting end of the twisted pair, but this may result in unsatisfactory performance, as measured by data error rate.
It is generally understood that transmitting FDDI signals over an unshielded twisted pair requires that the transmitted spectral width be decreased by the use of an encoding technique. A narrower transmitted spectrum results in lower signal attenuation, and therefore a longer transmission distance. The narrower spectrum also results in reduced crosstalk. More importantly, the narrower spectrum reduces overall electromagnetic emission, which allows driving voltages to be increased for improved performance, as reflected by some combination of lower error rate and longer transmission distance.
Several different solutions, in the form of different encoding schemes, have been proposed for FDDI transmission of unshielded twisted pairs. For completeness, they are briefly summarized in the following paragraphs.
Carrierless amplitude/phase encoding, known as CAP-32, maps five bits into thirty-two amplitude/phase symbols (with six amplitude levels) using a pair of superimposed waveforms generated from an orthogonal pair of bandpass filters. The principal disadvantage of the CAP encoding scheme is that it requires the use of elaborate digital signal processing apparatus. A similar scheme is known as QAM-32, except that it uses a 25 MHz carrier and its signal spectrum occupies a 50 MHz frequency range instead of the 25 MHz of CAP-32.
Two binary, one quaternary (2B1Q) encoding is a 4-level pulse amplitude modulation (4-PAM) technique. The code converts blocks of two consecutive binary digits into a single 4-level pulse for transmission. The scheme requires crosstalk suppression, scrambling of the data to spread the energy across the spectrum, and adaptive equalization to compensate automatically for channel characteristics as the channel length is changed.
8B10 code is an 8-level pulse amplitude modulation (PAM) scheme with low crosstalk immunity, and requires careful equalization.
Modified duobinary (MDB) code, used in the aforementioned patent to Ginzburg et al, is also known as PR-4 code. Its three-level format compresses the spectrum by about half. The spectrum has nulls at 0 and 62.5 MHz and the energy peak is at 32.5 MHz. The principal drawback of this scheme is that the use of two channels in each direction requires demultiplexing and multiplexing functions.
Another code known as QPR-4, or quaternary PR-4, has a spectral null at about 62 MHz and a spectral peak at about 17 MHz. It requires an analog-to-digital converter and digital signal processing based equalization and crosstalk suppression.
Predistorted NRZI coding is not a serious contender since it does not compress the frequency spectrum. NRZI stands for "nonreturn to zero, invert on ones." What this means is that a polarity transition in the signal represents a logical "1" and the absence of a polarity transition represents a logical "0." In predistorted NRZI coding, distortion is introduced in the transmitter to compensate for the channel frequency response. Since the spectrum is not compressed, the technique requires crosstalk suppression and careful scrambling.
Multi-Level Transmit 3-level (MLT-3) code is a 3-level variation on NRZI. A logical "1" is represented as a transition between adjacent signal levels. Each "0" input bit causes the same level to be repeated in the code, and each "1" input bit causes the code to advance by one level, in the sequence positive-zero-negative-zero. MLT-3 is another code that requires scrambling to spread its spectral energy across the frequency spectrum and avoid high-amplitude spectral lines that would present an emission problem for FCC certification.
Although a number of coding alternatives have been proposed, each has its drawbacks and none has yet emerged as an official or unofficial standard for FDDI transmission over a twisted pair of conductors. The present invention proposes a simpler alternative that solves many of the problems posed by other coding schemes, and provides a significant advantage in terms of electromagnetic emission.